Data represents a significant asset for many entities. Consequently, data loss, whether accidental or caused by malicious activity, can be costly in terms of wasted manpower, loss of goodwill from customers, loss of time and potential legal liability. To ensure proper protection of data for business and legal purposes, many entities back up data to a physical storage media such as magnetic tapes or optical disks. Traditionally, backup would occur at each machine controlled by an entity. As the sophistication of network technology increased, many entities turned to enterprise level backup in which data from multiple machines on a network is backed up to a remote media library. Centralized data backup allows storage problems to be identified at one location and has the advantage of increased efficiency.
One example of a media library commonly used in enterprise backup systems is a magnetic tape library. In a typical magnetic tape library, tapes are contained in cartridges and the tape library contains multiple cartridge slots in which tape cartridges can be stored. The tape cartridges are physically moved between cartridge slots and tape drives by a robot. The robot is controlled by access commands received from the host devices on the network. When specific data is required, the host device determines which cartridge slot contains the tape cartridge that holds the desired data. The host device then transmits a move-element command to the robot and the robot moves the tape cartridge.
In a SCSI tape library, devices that are part of the library are typically addressed by target number and logical unit numbers (“LUN”). Thus, each drive and robot of a tape library typically has a target number and LUN. Cartridge slots, on the other hand, are addressed by element numbers that are used by the robot to locate the slots. Because the robot also places tape cartridges in the drives, each drive is also associated with an element number. If multiple tape libraries are connected to a single device (e.g., a fibre channel to SCSI router), the tape libraries may be further addressed by bus number.
In current tape library systems, each tape library presents itself as an independent entity on the network. Each host in these systems maintains a view (i.e., a table of target numbers, LUNs and element numbers) of each of the tape libraries. Using this address information a host can format commands to the tape library to perform read/write, backup and other operations. In order to coordinate activities, hosts must cooperate with each other in issuing these commands. Enabling cooperation, however, requires reconfiguration of the hosts each time a new media library is added to the SAN. Moreover, to prevent conflicts between hosts, each host must typically use the same application to access a shared tape library. This can be inefficient as individual tape libraries cannot store data from multiple applications.
Prior art media libraries suffer additional shortcomings including lack of scalability. Currently, when an entity's need for storage outgrows the capabilities of its media library, the entity must, in many cases, purchase an additional independent media library. Because the independent media libraries present themselves as independent entities on a network, there is typically no single point of control for the libraries. Each host on the network will have access to each media library, often leading to conflicts.
One solution to this problem is an expandable media library in which physical units can be combined. In these systems, the robots which control the physical movement of media within a particular library can communicate with robots in connected libraries to transfer media such that the multiple libraries act as a single library. The use of multiple robots can allow some backup operations to continue even if one robot is offline. These systems, however, are often very expensive and require a large amount of space in single location so that the libraries can be connected together. Additionally, the entity installing such a system must have enough foresight to initially install a media library that is capable of expanding in this manner.
Another, more cost effective solution, is a set of modular media libraries that use a single robot. In these systems, each library installed after the initial library does not have its own robot, but, instead, is designed to stack on the initial library so that the robot from the initial library can move between the libraries. In essence, the libraries are combined into a single library using one robot. Again, however, this solution requires enough space in a single location and sufficient foresight on part of the installing entity to purchase a media library configured to allowing stacking. As a further deficiency, if the single robot experiences problems, the entity may not be able to perform any backup operations at all.